1. Field of the Invention
The present invention relates to an improvement of a light-triggered switching circuit such as a photo SCR (Silicon Controlled Rectifier) or a photo triac, either having light-driven PNPN elements.
2. Description of the Related Art
In general, light-triggered switching circuits comprise light-driven PNPN elements. Among these circuits are the light-activated SCRs shown in FIGS. 1 and 2, which are widely used as high-voltage relays wherein the current-supplying section and the control section must be insulated from each other. More specifically, the photo SCR shown in FIG. 1 comprises a PNPN element 21 and a resistor 30. The PNPN element 21 consists of a P-type emitter layer 22, an N-type base layer 23, a P-type base layer, an N-type emitter layer, an anode 26, a gate 27, and a cathode 28. The resistor 30 is used to adjust the sensitivity which the element 21 exhibits with respect to a light beam 29. The photo SCR illustrated in FIG. 2 is identical to the photo SCR shown in FIG. 1, except that a circuit 31 is provided in addition to a PNPN element 21 and a resistor 30. The circuit 31 comprises an N-channel MOSFET 32 and an PN-junction diode 33.
The operation of the photo SCR shown in FIG. 1 will be explained. When a light beam 29 is applied to the PNPN element 21 while a voltage is being applied to the anode 26, a current flows between the anode 26 the cathode 28. Whatever voltage is applied across the PNPN element 21, the photo SCR performs a switching operation. This photo SCR will, therefore, be referred to as "non-zero-crossing photo SCR" hereinafter.
The light-activated photo SCR shown in FIG. 2 is designed to perform a switching operation when the voltage applied between the anode 26 and the cathode 28 falls below a predetermined value (i.e., near 0 volt). The photo SCR will, thus, be referred to as "zero-crossing photo SCR" hereinafter. To be more precise, when the voltage applied between the gate and source of the N-channel MOSFET 32 falls below the threshold value (V.sub.TH) thereof, this MOSFET 32 is turned off. As a result, it is only the resistor 30 which is connected between the gate 27 and the cathode 28. In this case, the zero-crossing photo SCR becomes equivalent, in structure, to the non-zero-crossing photo SCR shown in FIG. 1. On the other hand, when the voltage applied between the gate and source of the MOSFET 32 rises above the threshold value thereof, the MOSFET 32 is turned on. The gate 27 and the cathode 28 are thus short-circuited Therefore, the carrier, which has been generated from the light 29 applied to the PNPN element 21, flows to the cathode 28 through the MOSFET 32. As a result of this, the zero-crossing photo SCR is not turned on.
All light-triggered switching circuits hitherto known are classified into the aforementioned two types, i.e., the zero-crossing photo SCR and the non-zero-crossing photo SCR. There has been provided no light-triggered switching circuits which performs the functions of both types of photo SCRs. Inevitably, the users must choose either a zero-crossing photo SCR or a non-zero-crossing photo SCR, in accordance with a specific use.